Could be a bad install.
On our TT the pump was mounted 14 feet horizontally from the tank. This caused all kind of priming problems and I often had air in the lines. It took lots of running to get it out and then after sitting the air would come back. Think about holding liquid in a straw while you have your finger on the top. Turn the straw horizontal and the liquid starts drizzling out even though your finger is still on the top.
The pump manufacturers say to have no more than 10 feet between the pump and tank but I can tell you it works better the closer you get.
I ended up moving my pump right over the tank and it works fantastic now.